Carcharhinus obscurus, the Dusky shark, is a common coastal-pelagic shark inhabiting tropical-warm temperate waters circumglobally. They range from near shore to depths of 400 meters. Maturing at 2.5-3.0 meters, duskys reach 4.2 meters in length and feed on large & small teleosts, sharks & batoids and invertebrates. (Ref. Compagno 1984, Compagno et al 2005, and FishBase.org.)

The dentition is cutting-clutching in design; cutting provided by broadened uppers with a serrate cutting-edge. Teeth number 13-15 per quadrate with one or two parasymphyseals (Garrick 1982: 122) displaying gradational dignathic heterodonty. Neither Garrick nor Compagno discuss tooth variations, however two forms appear present in extant and fossil individuals (BH pers. obs.):

  • Type I upper teeth are roughly triangular with a fairly straight root edge and a straight or wavy mesial cutting edge. The distal edge is more smoothly curved inward. The lower teeth are massive with the tips of the cusps erect or angled rearward and possessing fine serrations. (See Garrick 1982: 121, Fig. 56 illustration.)
  • Type II (possibly female) are triangular with a u-shaped root edge and smoothly, outwardly curved mesial and smoothly inwardly curved distal cutting edges. The finely serrated cusps of the lower teeth also smoothly curve distally.
    Ontogenetically, crown width increases at a greater rate than crown height (see figure ).

    Based on the accompanying (Type II) tooth-set (fig. ) from a large juvenile / young adult, the tooth-set would be described as follows.
    Uppers. Teeth are triangular, upright in the first position but otherwise more inclined distally. Although the crown's height/width ratio declines distally, the crown width increases through the 7-9th positions; the 4th & 5th positions tend to have the tallest crowns. The cutting edge is fully serrate with the basal portion of the crown more coarsely serrate, particularly in more anterior positions. In profile, the tooth is not particularly thick and the cusp labially curved, particularly in more posterior positions.
    Lowers. Teeth are proportionately smaller with a narrow, finely serrate, distally curved crown. In the first two positions, the cutting edge and serrations are limited to the apical portion of the cusp. Starting in the third position, the shoulders begin to display serrations; by the sixth position the cutting edge is complete. The tallest lower is in the third position and the broadest in the eighth. From a lateral perspective, the crown has a weak sigmoidal profile.

    Published Fossil Record

    Although very common in the Pliocene, there are few clear examples of this tooth-design in the published record; the most obvious examples include:

  • Kent (1994: 80-82) reported C. longimanus (POEY, 1861) from the Pliocene of the Chesapeake region. Although it is not possible to determine from his figures the precise nature of the tooth used to represent this taxon, it is likely to represent the most common Pliocene Carcharhinus of the area -- obscurus.
  • Müller (1999: 53, pl 7, figs 5-8) reported Pterolamiops longimanus from the Yorktown Formation (Pliocene, NC). Assuming there is an error in the published enlargement factor, most of these teeth represent positional variations of the extant C. obscurus; the angular mesial edge of figures 5a & b is a common ?pathology in this taxon.
  • Purdy et al (2001: 154) noted obscurus as the most common grey shark in the Yorktown (Unit 1-3).

    A reconstruction of isolated teeth (ref Fig. above) supports the argument that these teeth represent a Pliocene variation of the modern taxon. Purdy et al (2001) limited obscurus to the Yorktown. This tooth-design is occasionally recovered from Pungo River tailings at Lee Creek, it is impossible to be sure that these incidents always represent contaminates; first occurrence is noted herein as ?Upper Miocene rather than Lower Pliocene.


     

    References

    Compagno, L.,1984. FAO Species Catalogue, Vol 4, parts 1 & 2 Sharks of the World. United Nations Development Program.
    Compagno, L.,1988. Sharks of the Order Carcharhiniformes. Princeton University Press, Princeton, NJ. 486 pp , 35 plates.
    Compagno, L,, Dando & M., Fowler, S., 2005. Sharks of the World. Harper Collins, 368 p.
    FishBase.org Apr. 2009.
    Garrick, J., 1982. Sharks of the Genus Carcharhinus. NOAA Technical Report NMFS Circular 445. 194 pp.
    Kent, B., 1994. Fossil Sharks of the Chesapeake Region. Egan Rees & Boyer, Maryland. 146 pp.
    Müller, A. 1999. Ichthyofaunen aus dem atlantischen Tertiär der USA. Leipziger Geowissenschafteb, Leipzig, 9/10: 1-360.
    Purdy, R., Schneider, V., Appelgate, S., McLellan, J., Meyer, R. & Slaughter, R., 2001. The Neogene Sharks, Rays, and Bony Fishes from Lee Creek Mine, Aurora, North Carolina. In: Geology and Paleontology of the Lee Creek Mine, North Carolina, III. C. E. Ray & D. J. Bohaska eds. Smithsonian Contributions to Paleobiology, No 90. Smithsonian Institution Press, Washington D.C. pp. 71-202.

     

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